This invention relates to a system which although dispensing with the use of a costly and bulky gas chromatograph enables precise and reliable measurement and control of a gas mass flow to be achieved with a metering valve, even when the gas characteristics vary. Controlling a gas mass flow when the process requires a large ratio between maximum and minimum flows, with large variability in the pressure drop across the control valve, presents considerable difficulties, especially for fast control dynamics. Mass flow meters based on the Coriolis effect can achieve the required accuracy, reliability and measurement range, but typically provide insufficient measurement dynamics for those applications involving dynamically variable operating conditions. Another solution is achieved by using metering valves which potentially have all the necessary characteristics but require physical data regarding the gas, which have to be inevitably provided by an in-line gas chromatograph, if the gas composition varies. Said gas chromatograph introduces a considerable cost factor into the system, requires a supplementary gas feed giving rise to constructional complications and, in view of its structure, reduces the reliability and availability of the system.